Optical characteristics of rounded silver nanoprisms

We analyzed numerical optical characteristics of silver nanoprisms with rounded corners using the three-dimensional finite-difference time-domain method. The enhancement of the electric field was decreased from 240 to 13 times by introducing a large radius of curvature at the nanoprism corners such that it became a cylinder. This caused the optical multi-mode to change to single dipole mode. In the largest local electric field enhancement using the bowtie structure, which consisted of a pair of nanoprisms with rounded corners (the curvature radius and the gap distance were 8.66 and 1 nm, respectively), the electric field was enhanced by a factor of 360 at the hotspot. The bowtie structure that has non-zero curvature radii produces a larger electric field enhancement than does the single nanoprism without a curvature radius. Furthermore, the numerical simulation elucidates that the change of the curvature radius and the change of the gap distance have the same influence on the electric field enhancement.